1. Field of the Invention
The present invention relates to a filter for image sensor, and more particularly to a filter for image sensor for converting an optical signal not only in the visible but also in the non-visible region into electrical signal.
The invention is also suitably employed for a filter for image sensor for use in an image information processing device such as a facsimile, an image scanner, or a copying machine.
2. Related Background Art
In recent years, the uses of solid-state image pickup devices have been diversified, with new functions becoming needed. For example, in addition to having higher image quality and color picture for use in copying machines, they are required to recognize, reproduce and record an invisible image. Such image, or invisible image may be an image formed by ink having the characteristic of absorbing ultra-violet or infrared rays. The sensor technology for recognizing such images uses both a sensor for sensing the visible light and a sensor for sensing the invisible light.
In a copying machine for forming an image by sensing only visible light, an infrared cut-off filter has been disposed on the optical path between a solid-state image pickup device having color filters of red (R), green (G), and blue (B) provided on an IC chip of an image sensor such as a CCD and the original to enhance color separation ability. Accordingly, when reading and reproducing a printed image as the visible image, using an infrared absorbent ink, for example, one method can be conceived in which an optical system dedicated for infrared rays without an infrared cut-off filter is prepared, apart from the optical path of visible light, to image the infrared rays onto the solid-state image pickup device for the detection of infrared rays.
However, since the above-mentioned method is associated with a complex optical system, the present inventors have sought a new method which adopts a sensor in which a sensor for sensing the infrared rays and a sensor for sensing the RGB rays are integrated monolithically into one chip. However, this new method can not provide a conventional infrared cut-off filter in the optical path, because of its necessity of sensing the infrared rays.
Accordingly, a way of disposing an invisible light cut-off filter represented by an infrared cut-off filter must be improved, because otherwise one could not possibly make effective use of the advantages of the above new method.
For example, in Japanese Laid-open Patent Application No. 59-225564 or No. 62-174716, attempts have been made to integrate together a sensor for sensing the infrared rays and a sensor for sensing the RGB rays, but none of them have referred to the specific constitution of its filter.
On the other hand, there have been disclosed CCD image sensors capable of sensing the infrared rays and the visible rays in the specifications of U.S. patent application Ser. Nos. 174,444 (filed on Dec. 28, 1993), 174,453 (filed on Dec. 28, 1993), and 139,174 (filed on Oct. 21, 1993), assigned to the same assignee of the present application.
In particular, in a specification of U.S. patent application Ser. No. 174,444, there has been proposed a constitution (on-chip filter) having an iR cut-off filter provided on a chip of a CCD image sensor, but it has been recognized that the constitution of its filter must be further improved.
For example, it has been found that the use of a glass film containing metal ions, or a glass film having dye or pigment dispersed, as an on-chip filter for cutting off the infrared rays, might result in poor characteristics of the image sensor or less yield in manufacture.
As above described, regarding what construction should be taken to use an invisible light cut-off filter as the on-chip filter, it can be said that no detailed and sufficient research or examination has been made.